Telecommunications carriers (e.g., long distance providers) continually strive to increase the reliability of their communications networks. They do this, in part, by increasing the speed by which they can restore network operation following failure in one or more components of the network. A communications network consists of a collection of transmission links, also known as segments, that are interconnected at network nodes. The segments include transmission lines, fiber optic cables, microwave links, and other such transmission medium. Traffic is transmitted on the network from one endpoint to another endpoint through a current route or "trunk," which is a network path of segments that interconnect the endpoints. The network nodes may serve a variety of functions such as amplifying the network traffic for transmission down the next segment in the route or establishing an interconnection between two segments connected to the node (i.e., a switch). Each node is located at an installation, and several nodes may be located within a single installation. The restoration nodes can be controlled locally or from a remote computer system to connect or to disconnect segments that are connected to the node. Segments are connected to individual ports of a restoration node.
Unfortunately, the components (e.g., nodes and segments) of the communications network may occasionally fail. For example, a segment that is a buried fiber optic cable may fail as a result of being inadvertently severed by someone digging near the buried cable. If one or more of the cables fail, massive disruption of services to a large number of network customers could result. Therefore, telecommunications carriers strive to quickly and economically route the network traffic around such failed components by establishing a "restoral" route. A restoral route is a combination of segments between the endpoints that does not include the failed component. The establishing of a restoral route generally involves: (1) detecting that a component on the current route has failed, (2) identifying the location of the component, (3) selecting a restoral route to bypass the failed component, and (4) implementing the selected restoral route. The reliability of telecommunication networks depends in large part on the ability to detect such failures and implement the restoral route with minimal impact on network customers. A plan that identifies which restoration nodes are to be switched to bypass one or more specific failed components is called a "restoration plan."
Communications networks typically have excess capacity that can be used to bypass a failed component. The segments of a network that are currently being used to bear traffic are referred to as active segments, and the segments that are not being currently used to bear traffic (i.e., excess capacity) are referred to as spare segments. Restoral routes are implemented by identifying spare segments and incorporating certain of those spare segments into the network.
In common communications networks, segments are often physically collocated in larger components such as cables. In many instances, when one segment of the larger component fails, it is likely that the remaining segments of the larger component will also fail within a reasonably short amount of time. For example, if a cable catches on fire, the cable may end up being burned through, damaging all the segments within the cable. Initially, as the cable is being burned, perhaps only one or two segments on the periphery of the cable will be burned. As the heat intensifies and the fire progresses, the remaining segments will each be successively burned through and fail. It would be desirable for communications carriers to devise restoration plans in which spare segments that are not physically collocated with a failed segment are used to build a restoral route. The use of such spare segments would decrease the chance that the restoral route identified in the restoration plan will itself fail due to cascading failures of a cable that includes both the failed segment and the spare segment.